The Paleomagnetism of Sediments and Sedimentary Rocks: Importance and Reliability - Paleomagnetism of Sedimentary Rocks: Process and Interpretation - page 4

the concentration of magnetic minerals, magnetic par-
ticle grain size, and magnetic mineralogy of sedimen-
tary rocks as proxies of the paleoenvironment. Several
important topics have been written on environmental
magnetism (Thompson & Oldfi eld 1986 ; Evans &
Heller 2003) and the reader is referred to them for a
more complete treatment. Chapter 8 will however
introduce and cover rock magnetic cyclostratigraphy
in detail, which uses environmental magnetic princi-
ples. Rock magnetic cyclostratigraphy is an exciting
new use of mineral magnetic measurements that pro-
vides high-resolution chronostratigraphy for sedimen-
tary rock sequences.
Astronomically driven climate cycles are known
to be recorded by sedimentary sequences lithologi-
cally (Hinnov 2000), but the environmental mag-
netics of the rocks can be a very sensitive detector of
Milankovitch-scale climate variations. The rock mag-
netic record becomes particularly important when
climate-driven lithologic changes are diffi cult to iden-
tify in sedimentary rocks (Latta et al . 2006 ). Ultimately,
rock magnetic cyclostratigraphy can provide 20 kyr
resolution, much better than even the best magneto-
stratigraphy that records even the shortest geomag-
netic polarity chrons. Pioneering efforts by Ellwood
(e.g. Ellwood et al . 2010, 2011) looking at magnetic
susceptibility variations in stratigraphic type localties
have not focused exclusively on the magnetic response
to astronomically driven climate cycles. Measurements
that examine the concentration variations of only
depositional remanent magnetic minerals (magnetite
or hematite) can be more straightforward to interpret
than susceptibility measurements that respond to con-
centration variations of diamagnetic (calcite, quartz),
paramagnetic (iron-bearing silicates), and remanent
magnetic minerals. Concentration variations of rema-
nent magnetic minerals therefore have the potential to
provide cleaner records of global climate cycles, either
run-off variations from the continents or global aridity.

and sedimentary rocks can and do provide very high-
quality and accurate records of the Earth's magnetic
fi eld throughout geologic time. The intent of this topic
is not to give the impression that there are insurmount-
able problems with sedimentary paleomagnetic data.
Rather, the aim is to discuss some of the very important
inaccuracies that can arise in sedimentary paleomag-
netic data. These inaccuracies tend to be essentially
second-order effects, but prevent paleomagnetism from
achieving its full potential as an important tool for the
Earth sciences.
The evidence that young sediments can provide a
good record of the geomagnetic fi eld is plentiful. I will
show some examples, but it is by no means an exhaus-
tive list. In doing so I will also give an estimate of the
repeatability of these sedimentary records of the geo-
magnetic fi eld; this is not so much a rigorous measure
of the accuracy of the sedimentary paleomagnetic
recorder, but a way of estimating the precision of the
very best sedimentary paleomagnetic recorders. This is
probably the only way to understand the accuracy of
the paleomagnetism since, for most cases, the true
direction and intensity of the geomagnetic fi eld is not
known. In this approach I will rely on studies in the
scientifi c literature that report on multiple records
from cores that sample sediments, both lake and
marine, of the same age. The best records come from
the most recent sediments which have not yet been
appreciably affected by post-depositional processes,
chemical and physical, that can affect the magnetiza-
tion's accuracy and precision. Some of these studies
report the scatter in inclination and declination down-
core, others simply show plots of the agreement
between multiple records of the fi eld from which the
scatter can be estimated. I do not calculate statistical
parameters from these records but simply show, from
digitizing the plots, the range of scatter in inclination
and declination. The point of this exercise is to give the
reader a better feeling for the repeatability of paleo-
magnetic records of the fi eld and hence an estimate of
their accuracy.
Before embarking on an examination of multiple
records of the recent geomagnetic fi eld, we will briefl y
consider how sediments become magnetized parallel to
the Earth ' s fi eld. The process by which sediments
become magnetized is called the depositional or detrital
remanent magnetization (DRM) process. In this process
individual iron oxide, sub - micron - sized magnetic par-
ticles, typically magnetite (Fe 3 O 4 ), become oriented so
their magnetic moments are statistically biased toward

THE EVIDENCE FOR HIGH-QUALITY
PALEOMAGNETIC DATA FROM
SEDIMENTARY ROCKS

A main focus of this topic is the accuracy of sedimen-
tary paleomagnetic records, including some processes
that can cause inaccuracies and biases in sedimentary
paleomagnetism. The starting point of this discussion
must however be the understanding that sediments